The present invention relates to gas-permselective composite membranes and a process for the production thereof. More particularly, it relates to gas-permselective composite membranes comprising a thin layer of cross-linked structure provided on one side of a heat-resistant porous support, the thin layer being prepared by polymerizing tertiary carbon-containing compounds or tertiary organosilicon compounds by means of plasma. Also, the present invention relates to a process for producing the membranes.
In recent years, increasing number of extensive studies have been made to achieve separation and purification of fluid mixtures by the use of permselective membranes in place of conventional techniques such as distillation and low temperature processing, which are accompanied by changes in phase and consume a lot of energy as described in U.S. Pat. Nos. 4,230,463 and 4,264,338.
Separation and purification of fluid mixtures using membranes has already been put to practical use in several fields. For example, the conversion of sea water into fresh water, disposal of waste water from factories, and the concentration of foods have all been carried out on a commercial scale using appropriate membranes. These processes, however, are liquid-liquid separation and liquid-solid separation. However, gas-gas separation on a commercial scale is practically unknown.
It is difficult to commercially perform the separation of gases using a membrane (hereinafter sometimes referred to as "membrane-separation") because; (1) the permselectivity of conventional membranes is poor (More specifically, there is no suitable membrane which selectively allows specific gases to pass therethrough. While essentially blocking other gases making it possible to obtain high purity gas and therefore, it is necessary to employ a multi-stage process wherein the membrane-separation is performed repeatedly, which leads to increases in the size of the apparatus); and (2) the gas permeability is poor, which makes it difficult to process a large amount of gas. Furthermore, when the permselectivity of the membrane is increased, the gas permeability tends to be reduced. However, when gas permeability is increased, the permselectivity tends to be decreased. This makes it difficult to perform membrane-separation on a commercial scale.
In order to achieve commercial membrane-separation, various methods of producing improved membranes have been proposed. Typical examples include a method in which casting of a polymer solution is employed to produce an unsymmetrical membrane wherein the thickness of an active skin layer is made as thin as possible, and a method in which a super-thin membrane corresponding to the above active skin layer is prepared independently and stuck together to a porous support to form a composite membrane as described in U.S. Pat. Nos. 3,497,451, 4,155,793 and 4,279,855. These methods, however, fail to provide satisfactorily improved membranes although they are standard procedures to improve gas permeability. The reason for this is that there are no commercially available polymers or copolymers which meet all the required physical properties, e.g., permselectivity, gas permeability, heat-resistance, chemical resistance, and strength.
From the viewpoints of heat resistance and strength various materials can be chosen from porous polymerous materials now commercially available. Porous polysulfone, polyimide, and so forth may be used, but cellulose ester, polyvinyl chloride, polypropylene, polycarbonate, polyvinyl alcohol, etc. are not much preferred. In view of heat resistance and strength, a porous support made from a polytetrafluoroethylene is most preferred. Furthermore, it has the advantage that its chemical resistance is satisfactorily high.
With regard to gas permeability, the polytetrafluoroethylene is not suitable. Materials having satisfactory gas permeability include various rubbers such as silicone rubbers (e.g., dimethyl siloxane and phenyl siloxane), natural rubber, and polybutadiene. These rubbers, however, suffer from the serious defect of poor strength. It is possible to incorporate a silica filler into such rubber materials for the purpose of improving the strength. Incorporation of such fillers, however, is not preferred since it deteriorates gas permeability.
As a result of various investigations, it has been found that polymeric compounds containing a tertiary carbon atom in the recurring unit thereof have excellent gas permselectivity and, furthermore, those compounds containing a tertiary organic silicon in place of the above tertiary carbon atom are also excellent in gas permselectivity. However, these polymers are inferior in heat resistance, strength, and chemical resistance.